Silicone rubber

ABSTRACT

Silicone rubber containing a fumed silica doped with potassium by means of aerosol.

The invention relates to silicone rubber, to a process for itsproduction and to its use.

It is known to use hydrophobic fumed silica as filler in silicone rubber(DE 199 43 666 A1).

U.S. Pat. No. 6,331,588 describes LSR silicone rubbers which containfumed silicas as filler. In order to avoid the undesirable influence ofthe silanol groups on the mechanical properties of the silicone rubber,it is necessary according to U.S. Pat. No. 6,331,588 to render thesurface of the fumed silica hydrophobic.

According to the prior art, in the case of LSR (liquid silicone rubber),either a hydrophilic silica is rendered hydrophobic in situ and at thesame time exposed to very high shear forces so that the viscosity andthe flow limit can be lowered, or a silica that has already beenrendered hydrophobic is exposed to high shear forces for the samereason.

The invention provides a silicone rubber which is characterised in thatit contains as filler a fumed silica doped with potassium by means ofaerosol.

In an embodiment of the invention, the filler may be an oxide which hasbeen prepared pyrogenically by means of flame oxidation or, preferably,flame hydrolysis and which has been doped with from 0.000001 to 40 wt. %potassium, the BET surface area of the doped oxide being from 10 to 1000m²/g and the DBP absorption of the fumed oxide being undetectable orbeing less than 85% of the normal value for such fumed silica.

In a preferred embodiment of the invention, the amount of potassium usedfor the doping may be in the range from 1 to 20,000 (twenty thousand)ppm.

The fumed silicon dioxide (silica) doped with potassium by means ofaerosol is known from DE 196 505 00 A1.

If that low-structured fumed silicon dioxide is incorporated intosilicone rubber, totally novel properties of the silicone rubber result.

In a preferred embodiment of the invention, the silicone rubber may be aLSR silicone rubber.

In a further preferred embodiment of the invention, the silicone rubbermay be a HTV silicone rubber.

The filler can be prepared according to DE 196 50 500. On account of theadded potassium, the morphology of the fumed silicon dioxide is changed,so that a lower degree of intergrowth of the primary particles and hencea lower structure results.

For elastomer applications there are used polydimethyl-siloxanes whichhave molecular weights of from 400,000 to 600,000 and which are preparedwith the addition of regulators, such as hexamethyl- ordivinyltetramethyl-disiloxane, and carry corresponding end groups. Inorder to improve the vulcanisation behaviour and also the tear-growthresistance, small amounts (<1%) of vinyl groups are often incorporatedinto the main chain as substituents by the addition ofvinylmethyldichlorosilane to the reaction mixture (VMQ).

HTV silicone rubber is understood to mean water-clear, highly viscousself-deliquescing silicone polymers which have a viscosity of from 15 to30 kPas with a chain length of about 10,000 SiO units. As furtherconstituents of the silicone rubber there are used crosslinkers,fillers, catalysts, colouring pigments, anti-adhesives, plasticisers,adhesion promoters.

In hot vulcanisation, the processing temperatures are usually in therange of about from 140 to 230° C., whereas cold vulcanisation iscarried out at temperatures of from 20 to 70° C. In vulcanisation, adistinction is made between peroxidic crosslinking, additioncrosslinking and condensation crosslinking.

Peroxidic crosslinking takes place via a radical reaction mechanism. Theperoxides decompose under the action of temperature into radicals whichattach to the vinyl or methyl groups of the polysiloxanes and producenew radicals which are then bonded to other polysiloxane chains and thusresult in spatial crosslinking. The recombination of two radicals or theincreasing restriction on chain movability as the degree of crosslinkingincreases leads to termination of the crosslinking reaction.

In peroxidic crosslinking, different peroxides are used depending on theprocessing method (e.g. extrusion, injection moulding, compressionmoulding) in order to match the rate of crosslinking to theprocess-specific processing conditions. For example, very high rates ofcrosslinking are required for extrusion, and low rates of crosslinkingare necessary in the production of moulded articles by injectionmoulding or compression moulding, in order to avoid the onset ofcrosslinking during filling of the cavity.

The nature of the peroxide used also has an effect on the structure andhence on the physical properties of the vulcanate. Diaroyl peroxides(bis(2,4-dichlorobenzoyl) peroxide, dibenzoyl peroxide) crosslink bothvinyl and methyl groups. With dialkyl peroxides (dicumyl peroxide,2,5-(di-tert-butylperoxy)-2,5-dimethylhexane), on the other hand,vinyl-specific crosslinking takes place almost exclusively.

The Shore hardness of the vulcanate can be controlled to a certaindegree by the amount of peroxide in the mixture. As the amount ofperoxide increases, the Shore hardness increases owing to a higherdensity of crosslinking sites.

However, too large an amount of peroxide leads to a fall in ultimateelongation, tensile strength and tear-growth resistance. Depending onthe application, peroxidic crosslinking requires after-tempering of thevulcanates in order to reduce the permanent set and remove the cleavageproducts of the peroxides. In addition to the aromatic odour whichtypically occurs especially with dicumyl peroxide, the cleavage productsmay also lead to impairment of the physical properties of the vulcanates(e.g. reversion in the case of acid cleavage products).

In the case of fillers, a distinction is to be made between reinforcingand non-reinforcing fillers.

Non-reinforcing fillers are characterised by extremely weak interactionswith the silicone polymer. They include chalk, quartz powder,diatomaceous earth, mica, kaolin, Al(OH)₃ and Fe₂O₃. The particlediameters are of the order of magnitude of 0.1 μm. Their function is toraise the viscosity of the compounds in the non-vulcanised state and toincrease the Shore hardness and the modulus of elasticity of thevulcanised rubbers. In the case of surface-treated fillers, improvementsin tear strength can also be achieved.

Reinforcing fillers are especially highly disperse silicas having asurface area of >125 m²/g. The reinforcing action is attributable to thebond between the filler and the silicone polymer. Such bonds are formedbetween the silanol groups at the surface of the silicas (from 3 to 4.5SiOH groups/nm²) and the silanol groups of the α-ωdihydroxypolydimethylsiloxanes via hydrogen bridge bonds to the oxygenof the siloxane chain. The consequence of those filler-polymerinteractions are increases in viscosity and changes in the glasstransition temperature and the crystallisation behaviour. On the otherhand, polymer-filler bonds bring about an improvement in the mechanicalproperties, but may also result in premature crepe hardening of therubbers.

Talcum occupies a middle position between reinforcing andnon-reinforcing fillers. Fillers are additionally used for particulareffects. These include iron oxide, zirconium oxide or barium zirconatefor increasing heat stability.

Silicone rubbers may contain catalysts, crosslinkers, colouringpigments, anti-adhesives, plasticisers and adhesion promoters as furtherconstituents.

Plasticisers are required especially in order to establish a low modulusof elasticity. Internal adhesion promoters are based on functionalsilanes, which are able to interact on the one hand with the substrateand on the other hand with the crosslinking silicone polymer (useprincipally in RTV-1 rubbers).

Low molecular weight or monomeric silanol-rich compounds (e.g.diphenylsilanediol, H₂O) counteract premature crepe hardening. Theyprevent the silicone polymers from interacting too strongly with thesilanol groups of the filler, by reacting more rapidly with the filler.A corresponding effect can also be achieved by partially charging thefiller with trimethylsilyl groups (treatment of the filler withmethylsilanes).

It is also possible to modify the siloxane polymer chemically (phenylpolymers, boron-containing polymers) or to blend it with organicpolymers (butadiene-styrene copolymers).

Liquid silicone rubber (LSR) is virtually identical to HTV in itsmolecular structure, but its mean molecule chain length is lower by afactor of 6 and its viscosity is therefore lower by a factor of 1000(20-40 Pas). The processor has available two components (A and B) inequal amounts, which already contain the fillers, vulcanising agents andoptionally other additives.

As fillers there are used the same silicas and additives as in HTVmixtures. Because of the low viscosity of the starting polymer,particularly intensive incorporation and mixing in specially developedmixing units are required for homogeneous distribution. In order tofacilitate incorporation of the fillers and to avoid crepe hardening,the silica is rendered fully hydrophobic—mostly in situ during themixing operation and by means of hexamethyl-disilazane (HMDS, alsoHMDZ).

The vulcanisation of LSR mixtures is carried out by hydrosilylation,i.e. by addition of methyl hydrogen siloxanes (having at least 3 SiHgroups in the molecule) to the vinyl group of the polymer with catalysisby ppm amounts of Pt(O) complexes, the crosslinker and the catalystbeing in the separate components when supplied. Special inhibitors, e.g.1-ethynyl-1-cyclohexanol, prevent the premature onset of vulcanisationafter mixing of the components and establish a dropping time of about 3days at room temperature. The conditions can be adjusted in aconsiderable range via the concentration of platinum and inhibitor.

LSR mixtures are increasingly being used for the production ofelectrically conductive silicone rubber articles, because additioncrosslinking, in contrast to peroxide vulcanisation, which isconventional in the case of HTV, is not disturbed by furnace blacks (inHTV mixtures, acetylene black is preferably used). Conductive furnaceblacks are also easier to incorporate and distribute than graphite ormetal powders, with silver being preferred.

The silicone rubber according to the invention has the followingadvantages:

Tests in LSR (liquid silicone rubber) show that the doped oxides ofExamples 1 to 4 according to the invention (VP's 3739, 3650, 3740, 3744)exhibit markedly lower viscosities and flow limits in the liquidsilicone as compared with doped aerosils (fumed silicas) of equal orsimilar surface area. The markedly lower flow limits in particular areadvantageous, because very good flow behaviour is desirable whenprocessing liquid silicone rubber.

Using the hydrophilic potassium-doped oxides it is possible according tothe invention to use materials which, owing to their low structure,already have extremely low viscosities and flow limits and hence do nothave to be exposed to high shear forces during production. The saving ofenergy costs and material costs is advantageous for the user. Inaddition, the silicone rubbers according to the invention exhibitimproved optical properties in the form of high transparency.

In the case of HTV silicone rubber, the oxides doped with potassiumaccording to the invention also exhibit advantages in respect ofTheological properties. The Williams plasticity, a measure of viscosity,is markedly lower, especially after storage, than that of undoped fumedsilicas of comparable surface area. That effect is even more pronouncedin the case of prolonged storage. Over the entire test period of 22days, the Williams plasticities of the doped oxides (VP 3740, VP 3744)according to the invention are markedly lower than those of thehydrophilic comparison products (A 200, A 300). It is also surprisingthat, even when comparing VP 3740 with R 104, the Williams plasticitiesachieve a similar level. In the case of VP 3744, those values liebetween hydrophilic and hydrophobic AEROSIL.

The increase in viscosity during storage is referred to as crepehardening. For the processor, it is very important that this increaseshould be as small as possible, so that the silicone compounds remainprocessable even after storage or transportation and do not requireexpensive softening by rolling. The potassium-doped oxides exhibitmarked advantages in this respect compared with hydrophilic undopedfumed silicas.

EXAMPLES

Production of low-structured powders.

A burner arrangement as described in DE 196 50 500 is used.

Example 1

Doping with an Aerosol Prepared from a Solution of Potassium Chloride(3739)

4.44 kg/h of SiCl₄ are vaporised at about 130° C. and transferred to thecentral pipe of the burner according to DE 196 50 500. In addition, 3.25Nm³/h of hydrogen and 5.25 Nm³/h x₁: air and 0.55 Nm³/h of oxygen arefed into that pipe. The gas mixture flows out of the inner burner nozzleand burns in the combustion chamber of a water-cooled flame tube. 0.5Nm³/h of (jacket) hydrogen and 0.2 Nm³/h of nitrogen are additionallyfed into the jacket nozzle, which surrounds the central nozzle, in orderto avoid caking.

40 Nm³/h of air are additionally drawn into the flame tube, which isunder a slightly reduced pressure, from the surroundings.

The second gas component, which is introduced into the axial pipe,consists of an aerosol prepared from a 2.5% aqueous KCl-salt solution. Abinary nozzle which yields an atomisation output of 247 g/h aerosol isused as the aerosol generator. The aqueous salt aerosol is guided, bymeans of 3.5 Nm³/h of carrier air, through externally heated pipes andleaves the inner nozzle at a discharge temperature of 153° C. Thepotassium-salt-containing aerosol so introduced is brought into theflame and changes accordingly the properties of the fumed silica that isproduced.

After the flame hydrolysis, the reaction gases and the resulting fumedsilica doped with potassium (oxide) are drawn through a cooling systemby application of a reduced pressure, and the particle gas stream isthereby cooled to about 100 to 160° C. In a filter or cyclone, the solidis separated from the waste gas stream.

The resulting fumed silica doped with potassium oxide is obtained in theform of a finely divided white powder. In a further step, any adheringhydrochloric acid residues are removed from the doped silica attemperatures of from 400 to 700° C. by treatment with air containingwater vapour.

The BET surface area of the fumed silica is 107 m²/g. The content ofanalytically determined potassium oxide is 0.18 wt. %.

The preparation conditions are summarised in Table 1, the flameparameters are given in Table 2, and further analytical data of thesilica so obtained are shown in Table 3.

Example 2

Doping with an aerosol prepared from a solution of potassium chloride(3650).

The procedure is as indicated under Example 1:

4.44 kg/h of SiCl₄ are vaporised at about 130° C. and transferred to thecentral pipe of the burner according to DE 196 50 500. In addition, 4.7Nm³/h of hydrogen and 5.7 Nm³/h of air and 1.15 Nm³/h of oxygen are fedinto that pipe. The gas mixture flows out of the inner burner nozzle andburns in the combustion chamber of a water-cooled flame tube. 0.5 Nm³/hof (jacket) hydrogen and 0.2 Nm³/h of nitrogen are additionally fed intothe jacket nozzle, which surrounds the central nozzle, in order to avoidcaking.

25 Nm³/h of air are additionally drawn into the flame tube, which isunder a slightly reduced pressure, from the surroundings.

The second gas component, which is introduced into the axial pipe,consists of an aerosol prepared from a 9% aqueous KCl salt solution. Abinary nozzle which yields an atomisation output of 197 g/h aerosol isused as the aerosol generator. The aqueous salt aerosol is guided, bymeans of 4 Nm³/h of carrier air, through externally heated pipes andleaves the inner nozzle at a discharge temperature of 123° C. Thepotassium-salt-containing aerosol so introduced changes accordingly theproperties of the fumed silica that is produced.

After the flame hydrolysis, the reaction gases and the resulting dopedfumed silica are drawn through a cooling system by application of areduced pressure, and the particle gas stream is thereby cooled to about100 to 160° C. In a filter or cyclone, the solid is separated from thewaste gas stream.

The resulting fumed silica doped with potassium (oxide) is obtained inthe form of a finely divided white powder. In a further step, anyadhering hydrochloric acid residues are removed from the silica attemperatures of from 400 to 700° C. by treatment with air containingwater vapour.

The BET surface area of the fumed silica is 127 m²/g.

The preparation conditions are summarised in Table 1, the flameparameters are given in Table 2, and further analytical data of thesilica so obtained are shown in Table 3.

Example 3

Doping with an Aerosol Prepared from a Solution of Potassium Chloride(3740)

4.44 kg/h of SiCl₄ are vaporised at about 130° C. and transferred to thecentral pipe of the burner according to DE 196 50 500. In addition, 2.5Nm³/h of hydrogen and 7 Nm³/h of oxygen are fed into that pipe. The gasmixture flows out of the inner burner nozzle and burns in the combustionchamber of a water-cooled flame tube. 0.3 Nm³/h of (jacket) hydrogen and0.2 Nm³/h of nitrogen are additionally fed into the jacket nozzle, whichsurrounds the central nozzle, in order to avoid caking.

45 Nm³/h of air are additionally drawn into the flame tube, which isunder a slightly reduced pressure, from the surroundings.

The second gas component, which is introduced into the axial pipe,consists of an aerosol prepared from a 2.48% aqueous KCl salt solution.A binary nozzle which yields an atomisation output of 204 g/h aerosol isused as the aerosol generator. The aqueous salt aerosol is guided, bymeans of 3.5 Nm³/h of carrier air, through externally heated pipes andleaves the inner nozzle at a discharge temperature of 160° C. Thepotassium-salt-containing aerosol so introduced changes accordingly theproperties of the fumed silica that is produced.

After the flame hydrolysis, the reaction gases and the resulting fumedsilica doped with potassium (oxide) are drawn through a cooling systemby application of a reduced pressure, and the particle gas stream isthereby cooled to about 100 to 160° C. In a filter or cyclone, the solidis separated from the waste gas stream.

The resulting fumed silica doped with potassium (oxide) is obtained inthe form of a finely divided white powder. In a further step, anyadhering hydrochloric acid residues are removed from the silica attemperatures of from 400 to 700° C. by treatment with air containingwater vapour.

The BET surface area of the fumed silica is 208 m²/g. The content ofanalytically determined potassium oxide is 0.18 wt. %.

The preparation conditions are summarised in Table 1, the flameparameters are given in Table 2, and further analytical data of thesilica so obtained are shown in Table 3.

Example 4

Doping with an Aerosol Prepared from a Solution of Potassium Chloride(VP 3744)

4.44 kg/h of SiCl₄ are vaporised at about 130° C. and transferred to thecentral pipe of the burner of known construction according to DE 196 50500. In addition, 2.0 Nm³/h of hydrogen and 6.7 Nm³/h of air are fedinto that pipe. The gas mixture flows out of the inner burner nozzle andburns in the combustion chamber of a water-cooled flame tube. 0.3 Nm³/hof (jacket) hydrogen and 0.2 Nm³/h of nitrogen are additionally fed intothe jacket nozzle, which surrounds the central nozzle, in order to avoidcaking.

35 Nm³/h of air are additionally drawn into the flame tube, which isunder a slightly reduced pressure, from the surroundings. The second gascomponent, which is introduced into the axial pipe, consists of anaerosol prepared from a 2.48% aqueous KCl salt solution. A binary nozzlewhich yields an atomisation output of 246 g/h aerosol is used as theaerosol generator. The aqueous salt aerosol is guided, by means of 3.5Nm³/h of carrier air, through externally heated pipes and leaves theinner nozzle at a discharge temperature of 160° C. Thepotassium-salt-containing aerosol so introduced is brought into theflame and changes accordingly the properties of the fumed silica that isproduced.

After the flame hydrolysis, the reaction gases and the resulting fumedsilica doped with potassium (oxide) are drawn through a cooling systemby application of a reduced pressure, and the particle gas stream isthereby cooled to about 100 to 160° C. In a filter or cyclone, the solidis separated from the waste gas stream.

The resulting fumed silica doped with potassium (oxide) is obtained inthe form of a finely divided white powder. In a further step, anyadhering hydrochloric acid residues are removed from the doped silica attemperatures of from 400 to 700° C. by treatment with air containingwater vapour.

The BET surface area of the fumed silica is 324 m²/g. The content ofanalytically determined potassium oxide is 0.18 wt. %.

The preparation conditions are summarised in Table 1, the flameparameters are given in Table 2, and further analytical data of thesilica so obtained are shown in Table 3. TABLE 1 Experimental conditionsin the preparation of doped fumed silica Potassium salt Aerosol SiCl₄Primary air O₂ add. H₂ core H₂ jacket N₂ jacket Gas temp. solutionamount Air aeros. BET No. kg/h Nm³/h Nm³/h Nm³/h Nm³/h Nm³/h ° C.KCI-wt. % g/h Nm³/h m²/g 1 4.44 5.25 0.55 3.25 0.5 0.2 153 2.5 247 3.5107 2 4.44 5.7 1.15 4.7 0.5 0.2 123 9 195 4 127 3 4.44 7 0 2.5 0.3 0.2160 2.48 204 3.5 208 4 4.44 6.7 0 2.0 0.3 0.2 139 2.48 246 3.5 324Legend:Primary air = amount of air in the central pipe;H₂ core = hydrogen in the central pipe;Gas temp. = gas temperature at the nozzle of the central pipe;Aerosol amount = mass flow of the salt solution converted to aerosolform;Air aerosol = carrier gas amount (air) of the aerosol

TABLE 2 Flame parameters in the preparation of doped fumed silica Gammacore Lambda core vk_(norm) No. [−] [−] [m/sec] 1 2.77 1.01 20.8 2 4.001.00 25.9 3 2.13 1.17 21.6 4 1.71 1.40 20.0Legend:Gamma core = hydrogen ratio in the central pipe;Lambda core = oxygen ratio in the central pipe; for the precisecalculation and definition of gamma and lambda, see EP 0 855 368;vk_(norm) = discharge speed under standard conditions (273 K, 1 atm).

TABLE 3 Analytical data of the samples obtained according to Examples 1to 4 pH 4% Potassium DBP at 16 g aqueous content as weighed Bulk TampedBET dispersion K₂O amount in density density No. [m²/g] [−] [wt. %][g/100 g] [g/l] [g/l] 1 107 7.07 0.18 n.e.-p. 24 32 2 127 7.71 0.316n.e.-p. 31 42 3 208 6.66 0.15 234 19 25 4 324 6.35 0.18 305 17 22Legend:pH 4% sus. = pH value of the 4% aqueous suspension;DBP = dibutyl phthalate absorption;n.e.-p. = device does not detect an end-point.Low Structure:

A measure of the degree of structuring of a fumed silica is the dibutylphthalate absorption (DBP). The smaller the DBP number, the lower thestructuring (i.e. the degree of intergrowth) of the silica, i.e. of theprimary particles. However, because the DBP absorption itself is greatlydependent on the specific surface area (BET), the DBP must always begiven in conjunction with the specific surface area.

If the measuring device does not detect an end-point, the structure canbe assumed to be very low (DBP values markedly below 100 wt. %).

Normal value: A graph showing the relationship between DBP and BET foraerosil of “normal” structure is given in the series of documentsPigmente No. 11 from Degussa AG (page 30). That graph is to be definedas the “normal value” for fumed silica.

Accordingly, for Examples 1 and 2 of this invention, a DBP absorption ofabout 270 wt. % would be expected according to the graph given therein,but no end-point is detected, which indicates very low DBP values(markedly lower than 100 wt. %).

Testing of the Potassium-Doped Fumed Silicas in Silicone Rubber TABLE 4Analytical data BET DBP surface K₂O Tamped ab- Drying Batch area pHcontent density sorption loss no. [m²/g] value [wt. %] [g/l] [wt. %] [%]Ex. 1 VP 3739 107 7.07 0.18 32 — 1.1 Ex. 2 VP 3650 127 7.71 0.316 42 —1.7 Ex. 3 VP 3740 208 6.66 0.15 25 234 1.4 Ex. 4 VP 3744 324 6.35 0.1822 305 2.5

The products from Table 4 are tested in various silicone formulations(HTV, LSR). As comparison material there are used standard types ofaerosil having a comparable surface area (known from Ullmann'sEncyclopädie der technischen Chemie, Volume 21 (4th edition), page 462et seq. (1982).

HTV Silicone Rubber

Compounds containing 40 parts of silica and 6 parts of VHM (processingaid) are prepared on a twin roller according to a standard formulation.After 7 days, the mixtures are crosslinked with DCLBP peroxide.

The mechanical properties of the two potassium-doped samples accordingto Example 3 (VP 3740) and according to Example 4 (VP 3744) are slightlypoorer than those of the comparison samples (Table 5). TABLE 5Mechanical properties of the vulcanates and rheology of the compoundTensile Ultimate Tear-growth Rebound strength elongation resistanceHardness resilience Williams Product [N/mm²] [%] [N/mm] [Shore A] [%] 0d/7 d (VP 3740) 7.4 370 7.9 52 47 457/191 Ex. 3 Aerosil 200 8.5 470 9.856 48 830/339 (VP 3744) 8.6 445 9.9 60 48 820/233 Ex. 4 Aerosil 300 9.0455 12.5 64 52 864/546

The Williams plasticity of the compounds is determined afterincorporation and after 7 days' storage (Table 5). The compound becomessofter owing to the wetting of the silica which takes place during thestorage period. With prolonged storage, crepe hardening of the compoundoccurs, and the Williams plasticity increases again.

In the case of a normal hydrophilic pyrogenically prepared silicondioxide (Aerosil 200), the Williams plasticity falls markedly after astorage period of 7 days and then rises again sharply. By comparison,the product according to Example 3 (VP 3740) exhibits a markedly lowerinitial plasticity, which falls further after 7 days. As storagecontinues, the plasticity rises again here too, but to a lesser degreethan in the case of the undoped comparison material. The progression ofthe plasticity curve of the product according to Example 3 (VP 3740) canbe compared—at least in the initial region—not with that of an undopedAerosil 200 but with that of a hydrophobic Aerosil R 104 (FIG. 1).

In the case of the product according to Example 4 (VP 3744), theWilliams plasticity falls markedly after 7 days and then rises againcontinuously. By contrast, in the case of the undoped Aerosil 300, theWilliams plasticity remains at a constantly high level throughout thestorage period. The reduction after 7 days is very slight (FIG. 2).

LSR Silicone Rubber

In a planetary dissolver, 10% silica are incorporated at slow speed(50/500 min⁻¹ planetary mixer/dissolver plate) and then dispersed athigh speed (100/2000 min⁻¹) for 30 minutes.

After the incorporation, the mixture forms a highly viscous, almostsolid mass. After the 30 minutes' dispersion, the viscosity and the flowlimit fall markedly. While the product according to Example 3 (VP 3740)and the product according to Example 4 (VP 3744) still exhibit a veryhigh flow limit, the product according to Example 1 (VP 3739) and theproduct according to Example 2 (VP 3650) form a flowable formulation.

The undoped comparison silicas exhibit a markedly higher thickeningaction and a pronounced flow limit (Table 6). TABLE 6 Rheologicalproperties with 10% silica Viscosity Flow limit D = 10 s⁻¹ Silica [Pa][Pa · s] (VP 3739) 0 62 Ex. 1 Aerosil 90 482 97 (VP 3650) 0 60 Ex. 2Aerosil 130 866 138.5 (VP 3740) 533 98 Ex. 3 Aerosil 200 2176 260 (VP3744) 1535 286 Ex. 4 Aerosil 300 2370 291

The test is then repeated in the same manner using the product accordingto Example 1 (VP 3739) and according to Example 2 (VP 3650) and thecomparison samples Aerosil 90 and Aerosil 130.

When the 30 minute dispersion is complete, the silica content is raisedto 15% at slow speed (50/500 min⁻¹). The subsequent dispersion time(100/200 min⁻¹) of 30 minutes is interrupted after 5 minutes and after15 minutes in order for a sample to be taken. The rheological propertiesof those samples and at the end of the dispersion time are determined.

In the samples according to Example 2 (VP 3650), according to Example 1(VP 3739) and the comparison sample Aerosil 90, there are only slightdifferences in the viscosity, which falls markedly during the dispersiontime. The sample Aerosil 130, by contrast, has a markedly higherviscosity, the influence of the dispersion time is less too (FIG. 3).

The differences in the flow limit are markedly more pronounced (FIG. 4):

Although the product according to Example 2 (VP 3650) exhibits apronounced flow limit (=753 Pa) after 5 minutes, a flow limit is nolonger detectable after only 15 minutes.

The product according to Example 1 (VP 3739) exhibits a flow limit of1763 Pa after 5 minutes, which falls to 46 Pa after 15 minutes, andafter 30 minutes a flow limit can no longer be detected.

The two comparison samples exhibit a flow limit of 1975 Pa (Aerosil 90)and 3196 Pa (Aerosil 130) even after 30 minutes' dispersion.

The test is then continued by increasing the silica content to 20% atslow speed (50/500 min⁻¹). As in the preceding step, the subsequentdispersion time (100/2000 min⁻¹) of 30 minutes is interrupted after 5minutes and after 15 minutes in order for a sample to be taken. TABLE 7aRheological properties with 20% silica Flow limit Viscosity Silica [Pa]D = 10 s⁻¹ (VP 3739) 0 192 Ex. 1 Aerosil 90 1000 214 (VP 3650) 0 177 Ex.2 Aerosil 130 3068 615

At the end of the dispersion time, no flow limit can be detected in thecase of the samples according to Example 2 (VP 3650) and according toExample 1 (VP 3739). While the viscosity in the case of Aerosil 90 isonly slightly higher than that of the potassium-doped samples, the flowlimit is clearly pronounced. Aerosil 130 exhibits a value that is aboutthree times as high for both values.

In FIG. 5, the development of the flow limit is clear. The productaccording to Example 1 (VP 3739) exhibits a markedly higher flow limitafter 5 minutes than does the product according to Example 2 (VP 3650);no flow limit is detectable after 15 minutes in the case of bothsamples. Although in the case of the comparison samples Aerosil 90 andAerosil 130 the flow limit falls markedly starting from the very highinitial values (that of Aerosil 130 can no longer be determined), theflow limit is still very high after the dispersion.

The mixtures are then crosslinked. In the crosslinking, the standardformulation (optimised to a hydrophobic filler with a maximum dryingloss of 0.3%) is altered so that the amount of crosslinker (catalyst andinhibitor remain unchanged) was increased according to the higher dryingloss of the hydrophilic fillers used. TABLE 7b Mechanical and opticalproperties of the vulcanates with 20% silica Tensile UltimateTear-growth Rebound strength elongation resistance Hardness resilienceWilliams Silica [N/mm²] [%] [N/mm] [Shore A] [%] 0 d/7 d (VP 3739) 3.4220 2.5 41 62 17.8 Ex. 1 Aerosil 90 4.1 380 2.8 50 60 13.8 (VP 3650) 2.4290 2.0 34 57 21.7 Ex. 2 Aerosil 130 3.9 190 4.0 52 60 16.4

The two potassium-doped samples exhibit lower values for tensilestrength, tear-growth resistance and hardness. However, both samples aremarkedly more transparent than the comparison samples.

1. A silicone rubber composition comprising silicone rubber and a fumedsilica filler doped with potassium by means of aerosol.
 2. The siliconerubber composition according to claim 1, wherein the fumed silica filleris an oxide which has been prepared pyrogenically by flame oxidation orby flame hydrolysis and which is doped with from 0.000001 to 40 wt. % ofa doping substance, the BET surface area of the doped oxide being from10 to 1000 m²/g and the DBP absorption of the fumed oxide beingundetectable or being less than 85% of the normal value for that fumedsilica.
 3. The silicone rubber composition according to claim 1 whereinthe silicone rubber is a LSR silicone rubber.
 4. The silicone rubbercomposition according to claim 1 wherein the silicone rubber is a HTVsilicone rubber.